A special image using an optical anisotropy is used as a medium to identify (discriminate) authenticity, for example, securities, credit cards, and documents. In the use of these media, further improvement in a forgery-preventing property or a design property is desired.
As one of techniques which affect a visual state of the image or a state of the image itself, with the use of an optical anisotropy, there is a technique in which a cholesteric liquid crystal is used as an identification (discrimination) medium, and a reflected light of the cholesteric liquid crystal is detected by visual observation or an optical instrument.
The cholesteric liquid crystal is continuous. However, when the cholesteric liquid crystal is assumed to have a multilayered structure, each of molecular long-axis directions of the liquid crystal is parallel to each other and to the plane of the layer. Each layer overlaps with other layer by rotating inch by inch, thereby forming a sterically spiral structure. Due to this structure, the cholesteric liquid crystal has properties of selectively reflecting a specific color (wavelength). Namely, the cholesteric liquid crystal has characteristics of selectively reflecting to a circularly-polarized light of wavelength λ determined by the following formula:λ=n·p , wherein p represents a cholesteric pitch that is a distance for the direction factor of the spiral structure to come around by rotation of 360°; and n represents an average refractive index in each layer.
Due to the characteristics, the cholesteric liquid crystal exhibits a special kind of beautiful color. With respect to the image obtained by using a selective reflection of the cholesteric liquid crystal, an image perceived under a normal condition shows a predetermined change by a prescribed operation (putting a filter on the image, declining the image, or the like).
With respect to a low-molecular cholesteric liquid crystal, its cholesteric pitch changes depending on a temperature. In contrast, a high-molecular cholesteric liquid crystal selectively reflects a circularly-polarized light of a specific wavelength regardless of the temperature. For this reason, the high-molecular cholesteric liquid crystal is used in a conventional identification medium. As the identification medium in which the high-molecular cholesteric liquid crystal is used, for example, a forgery-preventing card described in JP-A-63-51193 (“JP-A” means unexamined published Japanese patent application) is known. However, there is a problem in which the high-molecular cholesteric liquid crystal takes time for alignment. Further, thermal resistance is also problematic in the production of the forgery-preventing card, since the aligned cholesteric material is rapidly cooled to fix the alignment. Further, the forgery-preventing card described in JP-A-63-51193 has no pattern and resultantly is not enough in terms of design properties and the like.
In the identification medium described in JP-A-2002-127647, a patterning of the high-molecular cholesteric liquid crystal is performed by etching, a pattern-like heat treatment, or a laser processing. However, the patterning by etching is subjected to the restriction of a substrate. The pattern-like heat treatment has a limited resolution of the pattern. Further, the laser processing has a limited processing speed.
In view of the above situations, a new technique in which a pattern of large area can be effectively formed is desired in the production of the identification medium using a high-molecular cholesteric liquid crystal.